A riflescope (also called a “scope” for short) is an optical weapon sight based on the Keplerian telescope. In its most basic form, a riflescope is a Keplerian telescope with a reticle or cross hairs added to mark the “point of aim”. A riflescope must be mounted on a rifle such that its aiming axis (optical axis) is aligned with the rifle's barrel axis as shown in FIG. 6. However, due to mechanical tolerances, it is hardly possible to achieve a precise alignment the first time a riflescope is mounted on a rifle. The trajectory of the bullet must also be considered so additional corrections in the vertical direction might be necessary depending on target distance. In view of these considerations, riflescopes include means for precise vertical (elevation) and horizontal (windage) adjustments to the point of aim. The process of adjusting or correcting the point of aim of a riflescope so that the point of aim and point of impact coincide is called “zeroing-in”. Since a rifle's bullet follows a ballistic trajectory, a rifle is zeroed-in at a known distance, say 100 m. At other distances, the rifle will shoot a predictable amount above or below the point of aim.
Early riflescopes were equipped with an external-adjustment system built into the mounts. Their point of aim was adjusted by mounts having micrometer windage and elevation mechanisms that moved the entire scope laterally and/or vertically. An advantage of external-adjustment scopes is that all the lens elements and the reticle remain centered on the same optical axis, providing highest image quality. The disadvantage of the external-adjustment mechanism is that the mounts must be able to support the entire weight of the riflescope under recoil. The external adjust mechanism is also bulky, heavy and susceptible to mud and dirt when used in the field.
Most modern riflescopes have an internal-adjustment mechanism using threaded screws mounted in turrets. The screws are connected to external knobs which are accessible by the shooter. Turning the knobs moves the reticle assembly inside the main tube against spring pressure. The knobs have clearly marked graduations around their circumference and many have a ball-detent system that clicks as the adjustment screws are turned. Each graduation or click represents a change in reticle position such that the point of aim is shifted by a small amount on the target. In modern riflescopes the graduations are commonly expressed as 1 cm at 100 m or 0.5 inch at 100 yards. The graduations may also be expressed in minutes of arc (MOA) or milliradians (mil). For the purpose of zeroing-in a rifle, 1 MOA is considered to be equal to 1 inch at 100 yards. Similarly, 0.1 mil corresponds to 1 cm at 100 m. These conventions are used in the present invention as well.
In recent years, several optical adjustment mechanisms have been invented by the present author to replace the mechanical adjustment mechanism described above. U.S. Pat. No. 8,749,887 issued on Jun. 10, 2014 describes a riflescope wherein a pair of movable wedge prisms are positioned between the objective lens and its focal plane. U.S. Pat. No. 9,164,269 issued on Oct. 20, 2015 and U.S. Pat. No. 9,644,620 issued on May 9, 2017 describe mechanisms that utilize tiltable and rotatable wedge prisms for adjusting the point of aim in a riflescope.
The present invention discloses a simpler design: the point of aim is adjusted by attaching one or more wedge prisms in front of the objective.